Install from file is not working
The file you should select will be a .py file, that can be found inside the .zip file. (Use WinRAR or 7Zip to open it) if the init.py doesn't work and it's the only file in there, you could have a corrupt download. What plugin are you trying to install?

Here are the specific installation options for installing the Mitsuba render engine:

First, download this file, then follow the following instructions:


  Unpack this file and move the resulting mitsuba folder into your
  Blender scripts/addons folder. After you do this, restart blender then go to user preferences, then find mitsuba in the addons tab and enable it. Then, you should be able to select
  Mitsuba as the currently active renderer (click either Cycles or Blender Render above the 3D view. 
  
  In the main scene tab, you
  will need to set the path to the main renderer executable (mitsuba.exe
  on Windows, Mitsuba.app/Contents/MacOS/mitsuba on MacOS,
  /usr/bin/mitsuba on Linux).


Then, to install the spaceship generator, go to this link and select "Clone or download" and download .ZIP

Then open the .ZIP and drag the spaceship_generator.py onto your desktop. Then, in blender, go to the addons tab and choose 'install from file'. then select the spaceship_generator.py through the dialog box. Afterwards, just enable the addon and you're good to go.